Frequency Division Multiplexing is used in a number of commercial wired and wireless applications, such as local-area network (LAN) products. It is a communications technique that divides a communications channel into a number of frequency bands. A sub-carrier carrying a portion of the user information is transmitted in each band. In OFDM (Orthogonal Frequency Division Multiplexing) systems, each sub-carrier is orthogonal with every other sub-carrier.
Typical commercial wireless applications include a transmitter with an antenna which transmits a signal through a channel where it undergoes reflection, absorption, attenuation, and takes multiple paths, as a result of which, the signal gets distorted when it reaches a receiver that is situated at a distance. OFDM systems are chosen for such applications because of their capability to operate in multi-path channels.
In an OFDM system, a transmitter typically codes, interleaves, and modulates a stream of information bits to obtain a stream of modulation symbols. The transmitter may transmit the OFDM symbols in frames, with each frame containing multiple OFDM symbols. The frames are further processed and transmitted to a receiver. The receiver then performs complementary processing and obtains the samples for each received OFDM symbols. The receiver typically performs frequency error estimation to determine the frequency error at the receiver. The frequency error may be due to a difference in the frequencies of the oscillators at the transmitter and receiver, Doppler shift and so on.
The performance of OFDM systems is generally very sensitive to the presence of frequency offsets. Typically, the transmitter and receiver clocks have to be synchronized and any resulting offsets can severely degrade the OFDM system performance. Generally, these offsets have to be estimated and compensated for reliable data recovery. The carrier frequency offset (CFO) can usually be estimated simply by calculating the correlation during known repeated preamble sequences or during the guard interval with a corresponding data portion. Further, in systems where the carrier and sampling clocks are derived from the same oscillator (i.e., locked clocks), for example IEEE 802.11a systems, sampling frequency offset (SFO) can be derived directly by scaling the CFO by a ratio of the clocks. However in some OFDM systems, the carrier and sampling clocks need not be derived from the same oscillator. In such instances, the carrier and sampling offsets will be independent and may need separate estimation algorithms and can be difficult and cumbersome to implement, estimate and compensate for reliable data recovery to improve OFDM system performance.